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diff --git a/xpcom/threads/TaskQueue.h b/xpcom/threads/TaskQueue.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef TaskQueue_h_
+#define TaskQueue_h_
+
+#include "mozilla/Monitor.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/Unused.h"
+
+#include <queue>
+
+#include "nsThreadUtils.h"
+
+class nsIEventTarget;
+class nsIRunnable;
+
+namespace mozilla {
+
+typedef MozPromise<bool, bool, false> ShutdownPromise;
+
+// Abstracts executing runnables in order on an arbitrary event target. The
+// runnables dispatched to the TaskQueue will be executed in the order in which
+// they're received, and are guaranteed to not be executed concurrently.
+// They may be executed on different threads, and a memory barrier is used
+// to make this threadsafe for objects that aren't already threadsafe.
+//
+// Note, since a TaskQueue can also be converted to an nsIEventTarget using
+// WrapAsEventTarget() its possible to construct a hierarchy of TaskQueues.
+// Consider these three TaskQueues:
+//
+// TQ1 dispatches to the main thread
+// TQ2 dispatches to TQ1
+// TQ3 dispatches to TQ1
+//
+// This ensures there is only ever a single runnable from the entire chain on
+// the main thread. It also ensures that TQ2 and TQ3 only have a single runnable
+// in TQ1 at any time.
+//
+// This arrangement lets you prioritize work by dispatching runnables directly
+// to TQ1. You can issue many runnables for important work. Meanwhile the TQ2
+// and TQ3 work will always execute at most one runnable and then yield.
+class TaskQueue : public AbstractThread
+{
+ class EventTargetWrapper;
+
+public:
+ explicit TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
+ bool aSupportsTailDispatch = false);
+
+ TaskDispatcher& TailDispatcher() override;
+
+ TaskQueue* AsTaskQueue() override { return this; }
+
+ void Dispatch(already_AddRefed<nsIRunnable> aRunnable,
+ DispatchFailureHandling aFailureHandling = AssertDispatchSuccess,
+ DispatchReason aReason = NormalDispatch) override
+ {
+ nsCOMPtr<nsIRunnable> r = aRunnable;
+ {
+ MonitorAutoLock mon(mQueueMonitor);
+ nsresult rv = DispatchLocked(/* passed by ref */r, aFailureHandling, aReason);
+ MOZ_DIAGNOSTIC_ASSERT(aFailureHandling == DontAssertDispatchSuccess || NS_SUCCEEDED(rv));
+ Unused << rv;
+ }
+ // If the ownership of |r| is not transferred in DispatchLocked() due to
+ // dispatch failure, it will be deleted here outside the lock. We do so
+ // since the destructor of the runnable might access TaskQueue and result
+ // in deadlocks.
+ }
+
+ // Puts the queue in a shutdown state and returns immediately. The queue will
+ // remain alive at least until all the events are drained, because the Runners
+ // hold a strong reference to the task queue, and one of them is always held
+ // by the target event queue when the task queue is non-empty.
+ //
+ // The returned promise is resolved when the queue goes empty.
+ RefPtr<ShutdownPromise> BeginShutdown();
+
+ // Blocks until all task finish executing.
+ void AwaitIdle();
+
+ // Blocks until the queue is flagged for shutdown and all tasks have finished
+ // executing.
+ void AwaitShutdownAndIdle();
+
+ bool IsEmpty();
+ uint32_t ImpreciseLengthForHeuristics();
+
+ // Returns true if the current thread is currently running a Runnable in
+ // the task queue.
+ bool IsCurrentThreadIn() override;
+
+ // Create a new nsIEventTarget wrapper object that dispatches to this
+ // TaskQueue.
+ already_AddRefed<nsIEventTarget> WrapAsEventTarget();
+
+protected:
+ virtual ~TaskQueue();
+
+
+ // Blocks until all task finish executing. Called internally by methods
+ // that need to wait until the task queue is idle.
+ // mQueueMonitor must be held.
+ void AwaitIdleLocked();
+
+ nsresult DispatchLocked(nsCOMPtr<nsIRunnable>& aRunnable,
+ DispatchFailureHandling aFailureHandling,
+ DispatchReason aReason = NormalDispatch);
+
+ void MaybeResolveShutdown()
+ {
+ mQueueMonitor.AssertCurrentThreadOwns();
+ if (mIsShutdown && !mIsRunning) {
+ mShutdownPromise.ResolveIfExists(true, __func__);
+ mTarget = nullptr;
+ }
+ }
+
+ nsCOMPtr<nsIEventTarget> mTarget;
+
+ // Monitor that protects the queue and mIsRunning;
+ Monitor mQueueMonitor;
+
+ // Queue of tasks to run.
+ std::queue<nsCOMPtr<nsIRunnable>> mTasks;
+
+ // The thread currently running the task queue. We store a reference
+ // to this so that IsCurrentThreadIn() can tell if the current thread
+ // is the thread currently running in the task queue.
+ //
+ // This may be read on any thread, but may only be written on mRunningThread.
+ // The thread can't die while we're running in it, and we only use it for
+ // pointer-comparison with the current thread anyway - so we make it atomic
+ // and don't refcount it.
+ Atomic<nsIThread*> mRunningThread;
+
+ // RAII class that gets instantiated for each dispatched task.
+ class AutoTaskGuard : public AutoTaskDispatcher
+ {
+ public:
+ explicit AutoTaskGuard(TaskQueue* aQueue)
+ : AutoTaskDispatcher(/* aIsTailDispatcher = */ true), mQueue(aQueue)
+ , mLastCurrentThread(nullptr)
+ {
+ // NB: We don't hold the lock to aQueue here. Don't do anything that
+ // might require it.
+ MOZ_ASSERT(!mQueue->mTailDispatcher);
+ mQueue->mTailDispatcher = this;
+
+ mLastCurrentThread = sCurrentThreadTLS.get();
+ sCurrentThreadTLS.set(aQueue);
+
+ MOZ_ASSERT(mQueue->mRunningThread == nullptr);
+ mQueue->mRunningThread = NS_GetCurrentThread();
+ }
+
+ ~AutoTaskGuard()
+ {
+ DrainDirectTasks();
+
+ MOZ_ASSERT(mQueue->mRunningThread == NS_GetCurrentThread());
+ mQueue->mRunningThread = nullptr;
+
+ sCurrentThreadTLS.set(mLastCurrentThread);
+ mQueue->mTailDispatcher = nullptr;
+ }
+
+ private:
+ TaskQueue* mQueue;
+ AbstractThread* mLastCurrentThread;
+ };
+
+ TaskDispatcher* mTailDispatcher;
+
+ // True if we've dispatched an event to the target to execute events from
+ // the queue.
+ bool mIsRunning;
+
+ // True if we've started our shutdown process.
+ bool mIsShutdown;
+ MozPromiseHolder<ShutdownPromise> mShutdownPromise;
+
+ class Runner : public Runnable {
+ public:
+ explicit Runner(TaskQueue* aQueue)
+ : mQueue(aQueue)
+ {
+ }
+ NS_IMETHOD Run() override;
+ private:
+ RefPtr<TaskQueue> mQueue;
+ };
+};
+
+} // namespace mozilla
+
+#endif // TaskQueue_h_